AbstractNon‐Newtonian viscosity could be explained by the Graessely entanglement viscosity theory modified by a frictional viscosity for moderately concentrated solutions of polydimethyIsiloxanes over a range of shear rates from lower to upper‐Newtonian flow. The expansion of a polymer molecule was kept under control by use of solvents with different dissolving powers of the polymer. The result shows that only the frictional viscosity is strongly related to the degree of expansion of a polymer molecule in solution and that Graessley's plots are independent of that. Extrapolation of the frictional viscosity versus concentration curves to 100wt.‐% show a common value of 0,09 Pa·s in spite of different degrees of expansion of the polymer molecules in solution. This common value was equal to the viscosity value for melt polymer with the same molecular weight as a polymer segment. It is concluded that the frictional viscosity is caused by solvent‐solvent, solvent‐polymer segment, and segment‐segment